Contact printing using high volume presses is commonly employed to print a large number of copies of an image. Contact printing presses employ various printing elements such as printing plates, printing sleeves, printing cylinders, and the like to apply colorants to a surface to form an image thereon. The surface can form part of a receiver medium (e.g. paper) or can form part of an intermediate component adapted to transfer the colorant from its surface to the receiver medium (e.g. a blanket cylinder of a press). In either case, a colorant pattern is transferred to the receiver medium to form an image on the receiver medium.
These printing elements are a form of recording media that typically undergo various processes to render them in a suitable configuration for use in a printing press. For example, exposure processes are used to form images on an imageable surface of a recording media that has been suitably treated so as to be sensitive to light or heat radiation. One type of exposure process employs film masks. The masks are typically formed by exposing highly sensitive film media using a laser printer known as an “image-setter.” The imaged film mask is placed in area contact with a sensitized recording media, which is in turn exposed through the mask. Printing plates exposed in this manner are typically referred to as “conventional printing plates.” Typical conventional lithographic printing plates are sensitive to radiation in the ultraviolet region of the light spectrum.
Another conventional method exposes media directly through the use of a specialized recording apparatus typically referred to as a plate-setter. A plate-setter in combination with a controller that receives and conditions image data for use by the plate-setter is commonly known as a “computer-to-plate” or “CTP” system. CTP systems offer a substantial advantage over image-setters in that they eliminate film masks and associated process variations associated therewith. Typically, a recording head within the CTP system is controlled in accordance with image data to selectively emit radiation beams to form image picture elements known as image pixels on a surface of a recording media. The radiation beams typically induce a physical or chemical change to an image modifiable surface of the recording media.
Various factors can adversely affect the quality of the images formed on recording media. This has led to a need for the establishment of various process controls for the required image forming actions. Typically, there are a number of imaging parameters that need to be optimally set to achieve a desired quality result. One important parameter is the level of radiation exposure provided on the recording media. Exposure is typically defined as the amount of radiant energy per unit area that impinges on the recording media during the imaging process. Depending on the recording media type, it may be necessary to control this parameter within a few percent or less.
This situation is further compounded in multi-beam recording apparatus in that each beam needs to impart a substantially equal exposure to the recording media so that various imaging errors or artifacts are not created. Unless it can be guaranteed that all beams in a multi-beam recording head have identical size and propagation characteristics, it may not be possible to perform a simple power or intensity balance because exposure has both a spatial component and a power or intensity component. While it may be possible to directly measure beam size, the measurement is quite complicated and accurate results are difficult to achieve. Systems exists which are well suited to beam analysis but they are usually in the form of stand alone equipment and are not necessarily suitable or cost effective for inclusion in a CTP system.
The pragmatic approach, which is commonly adopted, is to let the recoding media be the measurement tool. Since the human eye is sensitive to slight image variations, a trained operator can sometimes make a diagnosis of an imaged recording media and perform the required adjustments to the recording apparatus based on these observations. The use of densitometers, which are instruments that determine the optical density of an image element by measuring the intensity of radiation reflected or transmitted by the image element are sometimes also employed.
Conventional methods for picking a best or optimum set-point for a particular imaging parameter typically involve plotting a series of image strips, each of the image strips being formed in accordance with a particular imaging parameter value. The optical density of each of the image strips is measured using a densitometer and the imaging parameter value corresponding to a particular optical density value is selected. Unfortunately, for many cases, the optical density varies only a little as the imaging parameter value is varied and the accuracy of the densitometer may be limited in detecting these subtle differences. These issues can make it very difficult to accurately set the particular imaging parameter to an optimum value.
Whether using a densitometer or simply judging a recording media by eye, the process remains manual and requires intervention of trained personnel. As the use of recording apparatus such as CTP systems gains in popularity, techniques that can be employed to conveniently adjust various imaging parameters for optimum performance become increasingly important.
There is a need to provide improved methods and apparatus for setting up imaging parameters important in the process control of an image forming operation undertaken on a recording media.
There is a further need to reduce operator intervention in an image parameter set-up process, particularly with respect to making judgments on the subjective quality of test patterns.